U.S. patent number 6,023,209 [Application Number 08/675,931] was granted by the patent office on 2000-02-08 for coplanar microwave circuit having suppression of undesired modes.
This patent grant is currently assigned to Endgate Corporation. Invention is credited to Mark V. Faulkner, Clifford A. Mohwinkel, Edward B. Stoneham, Mark J. Vaughan.
United States Patent |
6,023,209 |
Faulkner , et al. |
February 8, 2000 |
Coplanar microwave circuit having suppression of undesired
modes
Abstract
Two or three conductor coplanar transmission lines and lossy
coplanar resistive films are formed on a surface of a substrate.
The resistive film dimensions and resistivity are selected to
suppress various spurious electromagnetic modes within and around
the substrate. The resistive films may be positioned along the
outer edges of the transmission lines or between the transmission
line conductors. The resistive film may have regular spaced
openings for producing an average resistivity different than that
of a continuous resistive film. In one embodiment, a signal
conductor has a serpentine shape and resistive film elements are
positioned between adjacent sections of the signal conductor. In
another embodiment, interdigitated resistive film elements extend
between transmission line conductors.
Inventors: |
Faulkner; Mark V. (Boulder
Creek, CA), Stoneham; Edward B. (Los Altos, CA),
Mohwinkel; Clifford A. (San Jose, CA), Vaughan; Mark J.
(Sunnyvale, CA) |
Assignee: |
Endgate Corporation (Sunnyvale,
CA)
|
Family
ID: |
24712537 |
Appl.
No.: |
08/675,931 |
Filed: |
July 5, 1996 |
Current U.S.
Class: |
333/238; 257/664;
257/778; 333/12; 333/247 |
Current CPC
Class: |
H01L
23/66 (20130101); H01P 1/162 (20130101); H01P
3/003 (20130101); H01L 2223/6627 (20130101); H01L
2924/0002 (20130101); H01L 2924/0002 (20130101); H01L
2924/00 (20130101) |
Current International
Class: |
H01P
3/00 (20060101); H01P 1/162 (20060101); H01P
1/16 (20060101); H01P 003/08 (); H01P 005/00 ();
H01L 029/40 () |
Field of
Search: |
;333/12,24R,27,33,81A,238,246,247 ;257/778,664 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Dylan F. Williams, "Damping of the Resonant Modes of a Rectangular
Metal Package", IEEE Transactions on Microwave Theory and
Techniques, vol. 37, No. 1, Jan. 1989, pp. 253-256..
|
Primary Examiner: Pascal; Robert
Assistant Examiner: Summons; Barbara
Attorney, Agent or Firm: Anderson & Adamson, LLP Steres;
George M.
Claims
What is claimed is:
1. A coplanar circuit structure for suppressing spurious modes
comprising:
an insulating substrate having a planar surface;
a coplanar transmission line including at least first and second
spaced-apart coplanar strip conductors mounted on the substrate
surface, the first and second conductors being spaced apart by a
first gap;
a first resistive film disposed on the substrate surface and
extending coplanar with, adjacent to and along a length of the
first conductor; and
a second resistive film disposed on the substrate surface and
extending coplanar with, adjacent to and along a length of the
second conductor;
the first and second resistive films being coupled to the first and
second conductors for attenuating spurious modes.
2. A coplanar circuit structure according to claim 1 wherein the
first conductor has a first signal conducting edge, the second
conductor has a second signal conducting edge spaced away from the
first conducting edge by the first gap, the first and second
conductors extend transversely in opposite directions away from the
first gap, the first conductor defines a third conductor edge
spaced away from the first conductor edge, the second conductor
defines a fourth conductor edge spaced away from the second
conductor edge, the first resistive film defines a first resistive
edge coupled to the third conductor edge, and the second resistive
film defines a second resistive edge coupled to the fourth
conductor edge.
3. A coplanar circuit structure as set forth in claim 2 further
comprising:
a third coplanar conductor separated from and spaced between the
first and second coplanar conductors thereby forming a coplanar
waveguide transmission line.
4. A coplanar circuit structure as set forth in claim 2
wherein the first and second conductors and the first and second
resistive films have a common length, Lr, between opposed ends;
the first resistive film has a third resistive edge spaced away
from the first resistive edge at least a distance Wr; and
the second resistive film has a fourth resistive edge spaced away
from the second resistive edge at least a distance Wr', the length,
Lr, and the widths, Wr and Wr' being selected such that spurious
modes are attenuated.
5. A coplanar circuit structure as set forth in claim 2 wherein the
coupling between the first resistive edge and the third conductor
edge is electromagnetic coupling.
6. A coplanar circuit structure as set forth in claim 2 wherein the
coupling between the first resistive edge and the third conductor
edge is conductive coupling.
7. A coplanar circuit structure as set forth in claim 2 wherein the
first resistive edge and the third conductor edge are spaced apart
by a second gap having a predetermined width.
8. A coplanar circuit structure as set forth in claim 2 further
comprising a third resistive edge of the first resistive film being
spaced away from the first resistive edge by a predetermined
width.
9. A coplanar circuit structure as set forth in claim 8 further
comprising a fourth resistive edge of the second resistive film
being spaced away from the second resistive edge by the same
predetermined width.
10. A coplanar circuit structure, as set forth in claim 8 in which
the spacing of the third resistive edge from the first resistive
edge is such that spurious signals selected from the group of
waveguide and cavity mode signals are attenuated by the first
resistive film.
11. A coplanar circuit structure, as set forth in claim 8 in which
the spacing of the third resistive edge from the first resistive
edge is such that spurious slab mode signals in the substrate are
attenuated.
12. A coplanar circuit structure as set forth in claim 8 wherein
the third resistive edge of the first resistive film is spaced away
from the first resistive edge about .lambda..sub.x /4 or greater,
where .lambda..sub.x is the wavelength of the spurious mode.
13. A coplanar circuit structure as set forth in claim 8 wherein
the third resistive edge of the first resistive film is spaced away
from the first resistive edge a distance on the order of magnitude
of about 1.125 mm or 0.045 inches or greater.
14. A coplanar circuit structure, as set forth in claim 8 in which
the spacing of the third resistive edge from the first resistive
edge is such that spurious microstrip mode signals having current
components tending to flow conductively in the first resistive film
will be attenuated.
15. A coplanar circuit structure, as set forth in claim 8 in which
the conductive coupling between the first resistive edge of the
first resistive film to the third conductor edge of the first
conductor is by contiguous conductive contact.
16. A coplanar circuit structure as set forth in claim 2 further
comprising:
a flip-chip active device having at least two terminals mounted on
the substrate surface;
at least one of the first and second conductors being connected to
at least one terminal of the flip-chip device.
17. A coplanar circuit structure as set forth in claim 16, further
comprising:
a third coplanar conductor disposed on the surface, the third
conductor coupled to the coplanar resistive films;
a second flip-chip active device connection mounted to the third
conductor;
one of the resistive films defining a decoupling slot having a
first end and a second end and a length therebetween, the slot
interposed between the first one of the device connections and the
second flip-chip active device connection, the first end disposed
laterally on one side of the first and second connections, the
second end disposed laterally on the other side of the first and
second connections, the slot length being sufficient to essentially
decouple the first and the second connections from undesired
signals therebetween.
18. A coplanar circuit structure for suppressing spurious modes as
set forth in claim 16 further comprising:
a third coplanar conductor separated from and spaced between the
first and second coplanar conductors thereby forming a coplanar
waveguide transmission line.
19. A coplanar circuit structure as set forth in claim 2 in which
the first resistive film has an average sheet resistivity matched
to the characteristic impedance of the spurious mode.
20. A coplanar circuit structure as set forth in claim 2 in which
the first resistive film has an average sheet resistivity of about
50 ohms/square.
21. A coplanar circuit structure as set forth in claim 2 in which
the first resistive film has an average sheet resistivity between
about 10 and 1000 ohms/square.
22. A coplanar circuit structure as set forth in claim 2, in which
the spacing of the first conductor edge and the third conductor
edge are sufficiently large such that coplanar mode signals of
wavelength .lambda..sub.s can propagate along the first conductor
edge and the second conductor edge with essentially zero current
component along the third conductor edge.
23. A coplanar circuit structure as set forth in claim 2 in which
the first resistive film is in the form of a mesh having a
predetermined ratio of resistive area to insulating area.
24. A coplanar circuit structure as set forth in claim 2 in which
the first resistive film is in the form of a mesh having a
resistive area to insulating area ratio such that the average sheet
resistance matches the characteristic impedance of the spurious
mode.
25. A coplanar circuit structure as set forth in claim 2 in which
the first resistive film is in the form of a mesh having a
generally rectangular aperture.
26. A coplanar circuit structure as set forth in claim 2 in which
the first resistive film is in the form of a mesh having an average
sheet resistance which matches the characteristic impedance of the
spurious mode.
27. A coplanar circuit structure as set forth in claim 2 in which
the width of the first resistive film between the first resistive
edge and the third resistive edge is more than about
1/4.lambda..sub.x, where .lambda..sub.x is the wavelength of the
spurious signal to be suppressed.
28. A coplanar circuit structure as set forth in claim 2 in which
the first resistive edge of the first resistive film and the third
conductor edge of the first conductor are spaced apart and
electromagnetically coupled across a second insulating gap.
29. A coplanar circuit structure as set forth in claim 2 having a
first gap width of about 0.05 mm or 2 mils.
30. A coplanar circuit structure as set forth in claim 2 wherein
the third conductor edge is spaced away from the first conductor
edge by a distance on the order of magnitude of about 0.125 mm or
0.005 inches.
31. A coplanar circuit structure as set forth in claim 2 wherein
the second conductor edge is spaced away from the first conductor
edge by a distance on the order of magnitude of about 0.025 mm or
0.001 inches.
32. A coplanar circuit structure as set forth in claim 1 wherein
the first resistive film is spaced apart from the first conductor
by a second gap having a width that is not greater than a distance
on the order of magnitude of about .lambda..sub.x /90, where
.lambda..sub.x is the wavelength of the spurious mode.
33. A coplanar circuit structure as set forth in claim 1 wherein
the first resistive film is spaced apart from the first conductor
by a second gap having a width that is not greater than a distance
on the order of magnitude of about 0.05 mm or 0.002 inches.
34. A coplanar circuit structure as set forth in claim 1 wherein
the first and second resistive films are configured as respective
first and second fingers extending respectively from the first and
second conductors toward the second and first conductors.
35. A coplanar circuit structure as set forth in claim 34 wherein
the first and second fingers and the first and second conductors
are configured to form a continuous meandering gap.
36. A coplanar circuit structure as set forth in claim 35 wherein
the first conductor includes a section extending laterally away
from the second conductor in a U-shaped loop, with the first and
second fingers positioned in the loop.
37. A coplanar circuit structure as set forth in claim 36 wherein
the first finger is contiguous with the first conductor on three
sides at the base of the loop.
38. A coplanar circuit structure as set forth in claim 37 wherein
the second finger extends from the second conductor into the loop
and is spaced from the first conductor and the first finger.
39. A coplanar circuit structure as set forth in claim 38 wherein
the first conductor has a meandering shape forming a plurality of
the U-shaped loops, and there are a corresponding plurality of the
first and second fingers disposed in the loops, the conductors and
fingers defining a first meandering gap.
40. A coplanar circuit structure as set forth in claim 39 further
comprising a third conductor extending parallel with the second
conductor, the first conductor being disposed between the second
and third conductors and forming a plurality of U-shaped loops open
alternately toward the second and third conductors, a plurality of
spaced-apart third and fourth resistive fingers, with each third
finger being contiguous with the first conductor on three sides at
the base of each loop open toward the third conductor and each
fourth finger extends from the third conductor into each loop open
toward the third conductor and is spaced from the first conductor
and the associated third finger, the first and third conductors and
the third and fourth fingers defining a second meandering gap.
41. A coplanar circuit structure as set forth in claim 40 further
comprising:
a flip-chip active device having an input signal terminal and an
output signal terminal and at least one ground terminal;
the second and third conductors forming respective first and second
coplanar ground conductor strips longitudinally disposed and spaced
apart from each other on the substrate, each ground strip having
respective adjacent opposed proximal and distal ends and opposed
inward facing edges, the proximal end of at least one ground strip
having a connection to the at least one device ground terminal;
the first conductor forming a coplanar signal conductor strip
defined on the substrate disposed between the inward facing edges
of the first and second ground conductors, the signal strip
extending between a proximal end and a distal end, the proximal end
located near the proximal ends of the two ground conductors and
having a coplanar connection to one of the device signal terminals,
and the distal end located near the distal ends of the ground
conductors, the signal strip being comprised of a succession of
continuously connected coplanar conductive segments forming the
U-shaped loops, including a plurality of longitudinal conductive
segments, and a plurality of lateral conductive segments, each
segment being of equal width between respective opposed sides;
the plurality of longitudinal segments being oriented parallel to
the ground conductors, the longitudinal segments being each of
equal first length between respective proximal and distal ends,
every other longitudinal segment having one of the opposed sides
facing outward to, and spaced away from, the inward facing edge of
the opposite ground strip by a width, Dg;
the plurality of lateral segments being of equal second length
between respective laterally opposed first and second ends, each
lateral segment being positioned orthogonally between the
respective inner edges of the ground conductors, the first ends and
opposed ends of the lateral segments being spaced away from the
respective inward facing edges of the opposed ground conductors by
the width, Dg;
a first one of the lateral segments being located proximally to the
proximal end of the central signal conductor strip;
a second one of the lateral segments being displaced distally from
the first one of the lateral segments by the first length of the
longitudinal segments, each successive lateral segment being
displaced distally from the preceding lateral segment by the first
length of the longitudinal segments;
a first one of the longitudinal segments being disposed between the
first and second lateral segment, the first one of the longitudinal
segments having the respective proximal end joined with the
adjacent first end of the first lateral segment and the distal end
joined with the adjacent first end of the second lateral
segment;
a second one of the longitudinal segments having the proximal end
joined with the opposed end of the second lateral segment and the
distal end joined with the adjacent opposed end of the third
lateral segment;
each succeeding longitudinal segment having the respective proximal
end joined with the respective opposite adjacent end of the
preceding lateral segment, the distal end of the succeeding
longitudinal segment being joined with the opposite adjacent end of
the succeeding lateral segment;
an outer edge of each longitudinal segment being spaced away from
the inward facing edge of the respective ground strip by the gap
width, Dg;
the second and fourth fingers forming a plurality of coplanar
lateral resistive film strips being defined on the substrate,
disposed orthogonally to the ground conductors, each one of the
resistive film strips having a lateral length, between a respective
inward facing end and a respective opposed outward facing end, each
lateral resistive film strip having a longitudinal width between
opposed lateral sides connecting the opposed inward and outward
facing ends, each one of the coplanar resistive film strips being
connected to the inner edge of one of the ground conductor strips,
each succeeding one of the coplanar resistive film strips being
connected to the opposite ground conductor strip of the preceding
one of the coplanar resistive film strips;
each one of the coplanar lateral resistive film strips being
disposed between a respective two of the adjacent lateral conductor
segments, each lateral side of the each one of the resistive film
strips being spaced away from the respective lateral segments by
the gap width, Dg, the inward facing end of the each one of the
coplanar resistive film strips being directed toward the opposite
ground conductor strip, each one of the coplanar resistive film
strips forming a lossy ground conductor along either side, adjacent
to a respective first portion of each of the respective adjacent
lateral segments;
the first and third fingers forming a plurality of coplanar
resistive film areas being defined on the substrate, each area
disposed between a respective pair of lateral conductor segments,
each area having a longitudinal resistive film edge facing the
inward facing end of the respective lateral resistive film strip,
the longitudinal resistive film edge being spaced from the
respective inward facing end by the gap distance, Dg, each
resistive film area having a periphery coincident with the inner
edge of each respective longitudinal strip and a respective second
portion of each of the respective adjacent lateral strips, the
resistive film area having a third lateral length between the inner
edge of the respective longitudinal conductor strip and the
longitudinal resistive film edge.
42. A coplanar circuit structure as set forth in claim 40, in which
the first, second, third and fourth fingers are of the same
resistivity.
43. A coplanar circuit structure as set forth in claim 42, in which
the resistivity is between about 10 and 1000 ohms/square.
44. A coplanar circuit structure as set forth in claim 42, in which
the resistivity is about 50 ohm/square.
45. A coplanar circuit structure as set forth in claim 34 wherein
the first and second fingers extend along longitudinally displaced
lengths of the first and second conductors.
46. A coplanar circuit structure as set forth in claim 45 wherein
the first and second fingers and the first and second conductors
are configured to form a continuous meandering gap.
47. A coplanar circuit structure as set forth in claim 46 wherein
the fingers have proximal ends adjacent to the respective
conductors and distal ends that extend adjacent to each other.
48. A coplanar circuit structure as set forth in claim 46 wherein
there are a plurality of longitudinally spaced first and second
fingers, the first fingers being interdigitated with the second
fingers.
49. A coplanar circuit structure as set forth in claim 48 further
comprising a third conductor, the first conductor extending
parallel to and between the second and third conductors, a
plurality of spaced-apart third fingers extending from the first
conductor toward the third conductor and a plurality of
spaced-apart fourth fingers extending from the third conductor
toward the first conductor, the third and fourth fingers being
interdigitated.
50. A coplanar circuit structure as set forth in claim 49 wherein
the first and third fingers are in alignment along the first
conductor.
51. A coplanar circuit structure as set forth in claim 50 wherein
the first and third fingers are formed of an integral resistive
film overlaying the first conductor.
52. A coplanar circuit structure
as set forth in claim 51 wherein the first conductor is a first
conductor strip extending longitudinally between a first end and a
second end;
and the second and third conductors are a pair of coplanar
conductive ground strips being positioned on the substrate
oppositely and distally adjacent to and electrically coupled with
the respective distal ends of the respective plurality of
interdigitated resistive fingers, whereby the circuit structure
functions as a lossy bias circuit structure.
53. A coplanar circuit structure as set forth in claim 52, in which
adjacent resistive fingers are spaced apart by about 0.025 mm or 1
mil.
54. A coplanar circuit structure as set forth in claim 52, in which
each first and third resistive finger has a length of about 0.2 mm
or 8 mils and a width of about 0.25 mm or 1 mil.
55. A coplanar circuit structure as set forth in claim 52, in which
each second and fourth resistive finger has a length of about 0.18
mm or 7 mils and width of about 0.025 mm or 1 mil.
56. A coplanar circuit structure as set forth in claim 52, in which
the first conductor strip extends longitudinally about 6.6 mm or
260 mils.
57. A coplanar circuit structure as set forth in claim 52, further
comprising:
an active device having an input terminal, an output terminal and
at least one ground terminal mounted to the substrate surface;
the second end of the first conductor strip being electrically
connected to at least one terminal of the active device;
one end of at least one of the ground strips being electrically
connected to a ground terminal of the active device.
58. A coplanar circuit structure as set forth in claim 57, wherein
the electrical connections are coplanar connections.
59. A coplanar circuit structure as set forth in claim 57, in which
the active device is a flip-chip active device.
60. A coplanar circuit structure as set forth in claim 1 wherein
the first and second resistive films extend along longitudinally
displaced lengths of the first and second conductors.
61. A coplanar circuit structure as set forth in claim 1 wherein
the first and second resistive films and the first and second
conductors are configured to form a continuous meandering gap.
62. A coplanar circuit structure as set forth in claim 1 wherein
the first resistive film is disposed in the first gap.
63. A coplanar circuit structure as set forth in claim 62 wherein
the second resistive film is also disposed in the first gap.
64. A coplanar circuit structure as set forth in claim 63 wherein
the first and second resistive films extend along laterally
adjacent lengths of the first and second conductors.
65. A coplanar circuit structure as set forth in claim 64 wherein
the first and second resistive films are spaced apart by a second
gap.
66. A coplanar circuit structure as set forth in claim 65 wherein
the first conductor has opposed sides and opposed ends, the opposed
ends each having respective connections,
and further comprising a third conductor and third and fourth
resistive films, the first and third resistive films contiguously
adjoining the respective opposed sides of the first conductor
and the second resistive film contiguously adjoining the second
conductor and the fourth resistive film adjacent to and spaced
apart from the third resistive film by a third gap, the fourth
resistive film contiguously adjoining the third conductor,
whereby the first conductor may be connected to provide a low DC
resistance for efficient transfer of DC or low frequency power from
an external supply to an active device while absorbing or
attenuating the transmission of high frequency signals between the
ends of the first conductor.
67. A coplanar circuit structure as set forth in claim 66, having
at least one connection configured for connection to a flip-chip
active device, the other connection being configured for connecting
to an external power supply.
68. A coplanar circuit structure as set forth in claim 66, wherein
the length of the first conductor and the widths of the first,
second, third and fourth resistive films and the second and third
gaps are selected to provide the desired attenuation of mm-wave
signals traveling along the transmission line.
69. A coplanar circuit structure for suppressing spurious modes
comprising:
an insulating substrate having a planar surface;
a transmission line including at least first and second
spaced-apart coplanar conductors mounted on the substrate surface,
the first and second conductors being spaced apart by a first gap;
and
a resistive film disposed on the substrate surface and extending
coplanar with, adjacent to and along a length of the first
conductor in the first gap;
the resistive film being coupled to the first conductor for
attenuating spurious modes.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally is directed to a structure and
method of suppressing undesired modes of electrical current
propagation, such as cavity, slab, surface wave and microstrip
modes, in coplanar conductor transmission lines and circuits. The
invention is specifically related to the use of defined coplanar
resistive patterns disposed adjacent to the perimeter of strip
conductors of coplanar transmission lines and circuits connecting
mm wave, flip-chip mounted, active devices.
2. Previous Art
Many applications of coplanar strip transmission line structures
are known. Some previous patents dealing with mode suppression are
shown below.
______________________________________ U.S. Pat. No. Inventor
______________________________________ 3,351,816 Sear et al.
4,045,750 Marshall 4,600,907 Greliman et al. 5,105,171 Wen et al.
5,225,796 Williams et al. 5,349,317 Notani at al.
______________________________________
All of these patents show coplanar strip transmission line
structures.
BACKGROUND
Coplanar circuit structures are referred to in the art in several
ways. For the purposes of this discussion coplanar circuit
structures include coplanar waveguide structures (CPW) and coplanar
slotline structures (CSL). Both CPW and CSL are characterized by a
conducting sheet, having longitudinal gaps or slots defined between
signal carrying conductor edges of the sheet which are adjacent to
each other. In theory, the sheet forms ground planes, (for both CPW
and CSL) disposed on either side of the slot (or slots), which
extend transversely toward infinity from either side of the signal
carrying slot or slots.
In practice, due to spurious modes that are set up by these
infinite or semi-infinite sheets, it is better that the conducting
sheet extends only a finite transverse distance on either side,
sufficient to behave essentially as a CPW or CSL. The actual
structure for the purposes of this application, then, appears more
like parallel strips of conductors separated by gaps. A more
precise nomenclature might be coplanar waveguide strip structures
and coplanar slot strip structures. Those skilled in the art
recognize the equivalence of the theoretical concept and practical
expression; therefore in this document the terminology CPW and CSL
will be understood to be structures having essentially those
characteristics.
A coplanar waveguide structure (CPW) has one or more closely spaced
but separated longitudinal coplanar strip signal conductors
positioned transversely between and separated from two adjacent
longitudinal coplanar ground conductors by respective gap widths.
RF signals are carried along the facing edges of the signal to
ground conductors. The ground conductors may be much wider than the
gaps between signal to signal or signal to ground.
A coplanar slotline structure (CSL) has two closely spaced coplanar
longitudinally extending conductors having facing edges having a
transverse gap therebetween, that is generally much smaller than
the lateral width of the conductors. An RF signal is carried along
the facing edges of the conductors.
MICROSTRIP MODES
Coplanar conducting strips on a substrate can develop a microstrip
mode in which an undesired potential difference exists along the
conductor strip with respect to a spaced away ground plane, where
the ground plane can be located on the opposite side of the
substrate or spaced above and/or below the substrate and the
coplanar strips. This is illustrated with reference to FIGS. 9 and
10. A typical coplanar strip transmission line structure (CPW) 50
consists of a central signal conducting strip 52 spaced apart by
gap distance, Dg, between two adjacent coplanar conductors 54. The
CPW 50 is used to interconnect millimeter (mm) wave RF circuit
components from DC to hundreds of GHz. The circuit components may
be amplifiers, oscillators, mixers, tuning and delay elements and
the like (not shown).
Discontinuities, and particularly asymmetrical discontinuities,
such as stub tuning elements, at any location along the CPW line
can initiate coupling of energy from the CPW mode into microstrip
mode propagation.
With reference to FIGS. 9 and 10, the conducting strips 52, 54 are
defined on one side or top of an insulating substrate 56. The CPW
50 and substrate 56 are typically mounted in a conductive housing
58. Housing 58 provides shielding to reduce radiation or
electromagnetic interference effects (EMI). The substrate 56 may be
typically mounted with the opposite side, or bottom of the
substrate 56 on or near the interior of one surface of the housing
58, or may be suspended between two opposed, interior surfaces.
The housing 58 may thus form a ground plane 60 on the opposite side
and above the substrate 56. The thickness of the substrate 56 is
generally large enough that the electromagnetic fields associated
with propagation of signals of wavelength .lambda..sub.s along the
CPW 50 are relatively uninfluenced by the presence of the ground
plane 60. Alternatively, the ground plane 60 may be a conductive
sheet formed on the side of the substrate 56 opposite the
conductors 52, 54, as shown in FIG. 1, and the housing 58 may be
made of an insulating material, such as plastic.
Longitudinal electric currents flow in the conductors 52 and 54 as
indicated in FIG. 9 and as shown in the plot of lateral sheet
current density, Jc, versus lateral distance, x, in FIG. 11. The
current density, Jc, associated with the desired CPW modes is a
maximum in the conductors 52, 54 near the inner facing edges
adjacent to gaps 53 between the conductors 52 and 54. The current
density, Jc, falls off rapidly with distance from the inner facing
edges of conductors 52-54 due to attraction between highly
concentrated opposite polarity charges (indicated by plus and minus
signs along the facing edges of conductors 52, 54.)
Undesired microstrip mode (MSM) electric fields represented in FIG.
10 as directed arrows, Ema (electric fields in air) and Ems
(electric fields in substrate), can propagate in the dielectric
substrate 56 between the CPW 50 and the ground plane 60 or in the
air dielectric between the CPW and the housing 58. Undesired cavity
modes having electric fields shown as Ew, may also propagate in the
housing 58. The current density distribution, Jc', from the
undesired MSM currents is indicated by dashed lines in FIG. 11 and
is concentrated toward the outer edges of the ground conductors 54
due to like charges along the outer edges of conductors 54 of FIG.
9 repelling each other. MSM return currents flow in the ground
plane 60. Undesired slab and dielectric modes also have electric
fields, Em, propagating within the substrate 56.
In FIG. 1 of Williams et al, U.S. Pat. No. 5,225,796 ('796) there
is shown a lossy resistive sheet 22 in the place of the ground
plane 60 as a means to suppress spurious MSM propagating through
the substrate 56. This method provides some improvement at the cost
of additional processing, i.e., coating the backside of the
substrate 56 with nichrome and the like. FIG. 2B of '796 shows a
resistive film 28 along the outer edge of a coplanar conductor
having a width dimension less than that of the conductor and much
less than the wavelength of the signal propagating along the
coplanar conductor. The narrow film 28 is not as effective as
desired in suppressing either surface modes propagating along the
bottom surface of the substrate 26 or MSM waves associated With the
metal chuck acting as a ground plane 30 for the assembly 24.
WAVEGUIDE AND CAVITY MODES
However, other spurious modes are not attenuated by the backside
lossy sheet. For example, waveguide and cavity modes may also exist
in the volume of the housing 58 on either or both the conductor
side or substrate side of the CPW 50 circuit pattern. Electric
field lines of cavity or waveguide modes (WGM) are indicated in
FIG. 10 by the dashed lines, Ew.
Waveguide or cavity mode suppression may be achieved by restricting
the width and/or height, Wa and Hw of the housing 58. This ensures
the waveguide is below cutoff for signals of wavelength
.lambda..sub.f greater than 2Wa or 2Hw. However, higher- and
lower-order modes may still not be suppressed sufficiently for high
frequency operation, especially in the frequency range above 20
GHz. Also, in the range of 40 GHz or higher, the minimum required
size of the waveguide or cavity 58 becomes so small (on the order
of about 0.15 inches or 3.8 mm) that it may be difficult to
incorporate all the desired circuitry. It also becomes more
expensive to control the dimensions of the waveguide 58 as
machining tolerances become a significant fraction of the waveguide
dimensions, Hw and Wa.
Both MSM and WGM may also exist on bias connections between power
supply points and active components in the CPW circuits. Any
coplanar strips used to connect DC power from one point on the
circuit to another separated point may have a structure which will
support undesired modes. Coplanar strips providing DC power
connections may also support CPW modes in circuit areas which are
not intended for signal propagation.
Up to now, undesired mode suppression often requires bonding of
wires, tabs or loops from one part of the circuit to another.
Discrete components may be added at particular points to try to
absorb or minimize unwanted modes. Spurious mode suppression using
such methods requires additional parts and additional labor for
assembly of high frequency coplanar circuits.
It would be an advantage to provide a method for suppressing MSM,
WGM and undesired CPW spurious modes over a wide frequency
range.
It would also be an advantage to provide a method for suppressing
modes while also reducing the parts count.
With reference to FIG. 12, there is shown, schematically, a
perspective view of an exemplary prior art CPW circuit structure
100. The structure 100 includes a series of active, mm wave
components A1, A2, A3, connected by CPW transmission line segments
T1, T2, T3 feeding one port of a mixer, X1. A second set of series
components A4, A5 are connected by CPW transmission line segments
T4, T5 to another port of the mixer, X1. The inputs to X1 are mixed
down and fed to a final IF amplifier A6 through CPW segment T6.
The structure 100 is enclosed in a housing 102 composed of three
sections 104, 106, 108, each section having a relatively long,
narrow aspect along the respective CPW circuit segment. Each
section has a lateral width dimension, Wa1, transverse to the CPW
circuit direction and a vertical height in elevation, Hw1. These
dimensions must closely follow the CPW circuit topology and must be
tightly controlled to suppress spurious waveguide or cavity modes.
The constraints of closely following the circuit topology and tight
control adds additional cost and complexity to the manufacture of
CPW circuits.
The relatively long and narrow aspect ratios of the sections 104,
106, 108 give the structure 100 relatively less structural strength
than would otherwise be possible with a more square aspect ratio.
Additional structural support is therefore required to make the
assembly 102 strong and robust. It would be an advantage to provide
a mode suppression method and structure which would allow the
manufacturer to relax the dimensions Wa1 and Hw1 for reduced cost
and improved manufacturing flexibility.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a structure and method
for attenuating and suppressing spurious microstrip modes in CPW
and/or CSL circuits connecting to mm-wave active devices.
It is another object of the invention to provide a structure and
method for attenuating and suppressing spurious waveguide or cavity
modes in CPW and/or CSL circuits connecting to mm-wave active
devices.
It is another object of the invention to provide a structure and
method for attenuating and suppressing spurious modes in bias
structures of CPW and/or CSL circuits connecting to mm-wave active
devices.
It is yet another object of the invention to provide a structure
and method for simultaneously attenuating and suppressing spurious
microstrip, slab, waveguide and cavity modes in CPW and/or CSL
circuits connecting to mm-wave active devices over a wide band of
frequencies with minimal addition of process and manufacturing
steps.
It is also an advantage provided by this invention, to relax the
constraints of size and shape of the waveguide or cavity enclosing
a CPW and/or CSL circuit connecting to mm-wave active devices to
reduce the cost and increase the robustness of the overall
assembly.
The desirable normal mode of functioning of coplanar circuits
connecting to active devices is with the separate and spaced apart
adjacent conductor strips having differing electrical potential
associated with conduction of electrical current along and between
them.
The microstrip mode of functioning is prevented by loading the side
or outermost strips (which are typically ground reference
conductors or strips extending along the length of the conductor).
This provides a resistive electrical path coupled to the microstrip
mode electromagnetic fields.
In one aspect of the invention, a coplanar circuit structure is
generally provided for suppressing spurious modes comprising a
transmission line including at least first and second spaced-apart
coplanar conductors mounted on a substrate surface, the first and
second conductors being spaced apart by a first gap. A resistive
film is disposed on the substrate surface and extending coplanar
with and along a length of the first conductor in the first gap.
The resistive film is coupled to the first conductor for
attenuating spurious modes.
In another aspect of the invention, a coplanar circuit structure is
provided for suppressing spurious modes which comprises a
transmission line including at least first and second spaced-apart
coplanar conductors mounted on the substrate surface, the first and
second conductors being spaced apart by a first gap. A first
resistive film extends coplanar with and along a length of the
first conductor, and a second resistive film extends coplanar with
and along a length of the second conductor. The first and second
resistive films are coupled to the first and second conductors for
attenuating spurious modes.
A CSL embodiment of the present coplanar spurious mode attenuating
and suppressing circuit structure invention includes an insulating
substrate having a planar surface upon which is defined a first
coplanar conductor. The first conductor has a signal conducting
edge defining a signal propagation direction. A second coplanar
conductor is also defined on the planar surface. The second
conductor has a second signal conducting edge facing the first
conductor signal edge. The first edge and second edge are spaced
apart by a gap, generally, although not necessarily of uniform
width. For example, the gap may be tapered.
The first and second coplanar conductors have respective third
edges outwardly disposed from the first and second signal
conducting edges. The respective third edges are spaced away from
the related first and second signal conducting edges. The spacing
of the respective signal edges and the corresponding third edges is
such that signals of wavelength .lambda..sub.s can propagate along
the gap between the first edge and the second edge with essentially
zero current component along the third edges.
A first and second coplanar resistive film of a predetermined sheet
resistance is defined on the planar surface. The resistive films
have a respective fourth edge coupled to the corresponding third
edge of the first and second conductors. The resistive films also
have respective distal fifth edges spaced away from the
corresponding fourth edges. The resistive films have a width
between the fourth and fifth edges sufficient such that signals
propagating in undesired modes along the signal propagation
direction are attenuated and suppressed through the electromagnetic
field of the first and second coplanar conductors to the respective
fourth edges of the resistive films.
The coplanar mode suppression resistive structures of this
invention are particularly suited for signal interconnections and
bias connections to flip-chip mounted active components operating
in the mm-wave region.
In an alternative embodiment of the present invention, a spurious
mode suppressing circuit structure as summarized above is defined
with the spacing of the respective fifth edges from the
corresponding fourth edges such that spurious microstrip mode
current components tending to flow conductively in and near the
fifth edges of the resistive films and standing wave voltage
components will be attenuated by the resistive films. The spurious
microstrip mode signals associated with such current components
will be suppressed by the coupling to the respective resistive
films.
In another embodiment of the present invention of a coplanar
spurious mode suppressing circuit structure, the spacing of the
respective fifth edges from the corresponding fourth edges is such
that spurious waveguide or cavity mode signals in a waveguide or
cavity volume above the CPW structure are attenuated by the
resistive film.
In yet another embodiment of the present invention the resistive
files have a width between the fourth and fifth edges sufficient
such that undesired coplanar signals of wavelength, .lambda..sub.x,
sufficiently longer than a predetermined value and propagating
along the signal propagation direction are attenuated by the
current components conductively coupled through the first and
second coplanar conductors to the respective fourth edges of the
resistive films.
The resistive film may be defined having an average sheet
resistivity of between about 10 ohms/square and about 1000
ohms/square. A resistive film having a sheet resistance about equal
to the characteristic impedance of an undesired incident wave will
be effective in attenuating or absorbing such a wave. For example,
in otherwise unused areas of the substrate, and in specific areas
of the substrate enclosing active devices, the effective sheet
resistivity may be about 50 ohms/square.
Other values of sheet resistivity may be selected to attenuate
other modes having different characteristic impedance. The sheet
resistivity may be varied along the film to provide attenuation of
different modes at different locations.
The resistive film may also be defined as a sheet having a
plurality of apertures therein. The apertures may be arranged in a
regular array, e.g., forming a mesh of resistive material. The mesh
may be formed with a periodically repetitive pattern. The patterned
mesh may have a predetermined ratio of open, insulating area to
covered, resistive area, R.sub.oc. Through appropriate patterning
of a resistive film as a mesh, the resistive film having low
intrinsic resistivity can be made to effectively function as a
sheet of higher average resistivity for wavelengths of a dimension
greater than several multiples of the pattern period. This provides
a method for varying the average sheet resistance at different
points on the circuit, without changing the intrinsic sheet
resistance of the resistive film.
The apertures may also be arranged in an aperiodic, irregular, or
semi-random arrangement, as desired, to present different average
sheet resistance in different areas. An aperiodic, semi-random
pattern could be used to attenuate undesired modes having a broad
range of characteristic impedances over a wide frequency range.
An appropriate effective sheet resistance of the resistive film on
the surface of the circuit results in strong absorption of energy
from waveguide, slab or cavity modes that are incident on the
resistive mesh. The mesh structure therefore can attenuate and
suppress both microstrip and waveguide or cavity modes.
The insulating-to-resistive-area-ratio of the mesh may be made
different at various locations along the substrate. The area ratios
may be selected to present a different average sheet resistance at
different locations on the substrate for attenuating and
suppressing different spurious modes at the varied locations.
The mesh may be defined with a pattern of openings in a rectangular
grid, or an array of circular holes or the like, where the openings
are sufficiently smaller than the wavelength of the spurious signal
to be suppressed.
The width of the resistive film between the fourth edge and the
fifth edge is generally more than about 1/4.lambda..sub.x, where
.lambda..sub.x is the wavelength of the spurious signals to be
suppressed.
Very-low-frequency modes are also suppressed by configuring the
resistive film with additional large areas, each of which is
defined herein as a "sea of resistor", (SOR). This provides an
effective lossy structure enclosing the circuit patterns of
interest but not significantly coupled to the desired signals
propagating along the CPW transmission paths.
BRIEF DESCRIPTION OF THE DRAWINGS
For a further understanding of the objects and advantages of the
present invention, reference should be had to the following
detailed description, taken in conjunction with the accompanying
drawings, in which like parts are given like reference numerals and
wherein;
FIG. 1 illustrates an embodiment of a CSL structure including two
mode suppression resistive films for connection to flip-chip active
devices in accordance with this invention.
FIG. 2 depicts an embodiment of a CPW structure including to mode
suppression resistive films for connection to flip-chip active
devices in accordance with this invention.
FIG. 3 is a graph of current density vs distance taken along the
line 3--3 of FIG. 2.
FIG. 4 is a cross sectional elevation view of the coplanar mode
suppression structure of FIG. 2 in accordance with this invention
taken along the line 3--3 and enclosed in a waveguide or cavity
housing.
FIG. 5 is a perspective view of a mode suppression CPW structure
having a resistive film formed into a mesh in accordance with
another aspect of this invention.
FIG. 6 is a plan view of an exemplary mesh construction for one
embodiment of the present invention.
FIG. 7 illustrates a perspective view of a CPW circuit structure
having resistive mode suppression films connected to flip-chip
active devices in accordance with this invention.
FIG. 8 is a detailed plan view of an inset portion of FIG. 7.
FIG. 9 is a plan view of a prior art CPW structure.
FIG. 10 is a cross sectional view of the prior art structure of
FIG. 9 taken along the line 10--10.
FIG. 11 is a graph of current density for the prior art CPW
structure of FIG. 9.
FIG. 12 is schematic perspective view of a prior art CPW structure
enclosed in a waveguide or housing.
FIG. 13 is a schematic of a plan view of a lossy line, signal
absorbing CPW structure in accordance with this invention.
FIG. 14 is a cross section of the structure of FIG. 13 taken along
the line 14--14.
FIG. 15 is a plan view of an alternative signal absorbing structure
in accordance with this invention.
FIG. 16 is an equivalent circuit schematic of the signal absorbing
structure shown in FIG. 15.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
With reference to FIG. 1, there is shown a first embodiment of a
coplanar spurious mode attenuation and suppression structure 200 in
accordance with the present invention. The structure 200 is shown
as a coplanar slotline structure.
The structure 200 includes an insulating substrate 202, having a
thickness ts made from a low loss dielectric such as glass-cloth
teflon, silicon, alumina, beryllia, aluminum nitride, quartz, glass
and semi-insulating GaAs, or the like. The substrate 202 has a
planar surface 204 opposite from backside layer 229 and upon which
is defined a longitudinally extending first conductor 206. The
conductor 206 defines a first signal conducting edge 208. A second
conductor 210 is disposed on the surface 204, coplanar with the
conductor 206. The conductor 210 defines a second signal conducting
edge 212 spaced away from the first conducting edge 208 by a first
gap 214 having a width Wg.
At least one of the conductors 206, 210 are connected to a terminal
of a flip-chip active device (not shown) mounted on the substrate
surface 204. Interconnection of the coplanar spurious mode
suppression structures of this invention with flip-chip active
devices is described below.
The conductors 206 and 210 extend transversely in opposite
directions away from the gap 214. The conductor 210 defines a third
conductor edge 216 spaced away from the second conductor edge 212
by a distance Wc2.
A first resistive film 220 is disposed on the surface 204 coplanar
with the conductors 206 and 210. The coplanar resistive film 220
defines a first resistive edge 222 parallel to and spaced away from
the third conductor edge 216. The film 220 defines a third
resistive edge 224 spaced at a distance Wr away from the first
resistive edge 222.
The width Wc1 and the spacing Wg are selected to allow coplanar
mm-wave signals of wavelength .lambda..sub.s at a signal frequency
f.sub.s to propagate along the inner facing conductor edges 208 and
212. For wide band transmission of signals at mm-wavelengths, for
example, f.sub.s, between about 20 and 40 GHz, Wc1 will be on the
order of magnitude of about 0.125 mm (0.005 inches) and Wg will be
on the order of magnitude of about 0.025 mm (0.001 inches).
The degree of attenuation of undesired modes depends on the
specific dimensions and dielectric constant of the substrate 202,
the wavelength of the undesired modes and the length, Lr, of the
longitudinal conductors 206, 210 and the coupled resistive film 220
and width, Wr, of the resistive strip 220. For effective
attenuation and suppression of unwanted MSM of wavelength,
.lambda..sub.x, in accordance with the present invention, the
width, Wr, of the resistive film 220 is selected to be about
.lambda..sub.x /4 or about 1.125 mm (0.045 inches) or greater over
a length, Lr of about one wavelength, or about 4.5 mm (0.18 inches)
or greater at a frequency of about 40 GHz.
It is contemplated that as the width, Wr, of the resistive films is
reduced, the length, Lr, of the longitudinal conductors 206, 210
and coupled resistive films 220, 220' must increase to achieve the
desired attenuation of spurious modes. The relationship between Wr
and Lr for a given attenuation depends on the type of coplanar
transmission line, the type and thickness of substrate material,
the type of spurious mode, the film sheet resistance and the
wavelength of the mode.
The resistive film edge 222 and the conductor edge 216 may be
spaced apart by second gap 223 having width Wg2. The resistive film
edge 222 and the conductor edge 216 are sufficiently close to be
electromagnetically coupled, or may be in coincident contact, being
conductively coupled.
Also shown on FIG. 1, is an alternate configuration for the mode
suppression structure 200. The coplanar conductor 206 includes a
fourth conductor edge 216' spaced away from the first conductor
edge 208 by a distance Wc1. A second coplanar resistive film 220'
is disposed adjacent to the conductor 206. The second film 220'
defines a second resistive edge 225 parallel to and spaced away
from the conducting edge 216' by a third gap 223' having width Wg2.
The second film 220' defines a fourth resistive film edge 224'
parallel to and spaced away from the second resistive film edge 225
by the distance Wr'.
The two conductors 206, 210 may be made transversely symmetrical
about the edges 208, 212 by selecting the widths Wc1 and Wc2 to be
equal. The two resistive films 220, 220' may be symmetrically
disposed about the coplanar conductors 206, 210 minimizing the
initiation of undesired propagation modes.
The edges 224, 224' of the resistive films 220, 220', may be
disposed laterally away from the conductors 206, 210, by a distance
sufficiently great, such that the films 220, 220' cover any unused
portion (not shown) of the substrate surface 204, thereby providing
a broad area of resistive film (SOR) for absorbing undesired modes;
e.g., slab, cavity, waveguide, microstrip, and the like.
In preferred embodiments of this invention, a flip-chip active
device (not shown) having at least two terminals may be mounted on
the substrate 204. One or more of the device terminals (not shown)
may be connected to at least one of the first or second signal
conductors 206, 210. A more detailed description of an active
device embodiment of the present invention is presented below.
With regard to FIG. 2, there is shown an alternate CPW spurious
mode attenuation and suppression structure 231 in accordance with
the present invention. The CPW structure 231 is connected with a
flip-chip mounted device (not shown) in accordance with the present
invention. The structure 231 includes elements having the same
function as, and numbered the same as, elements in FIG. 1, with
additional elements herein described. A third conductor 230 is
defined on the surface 204 of the substrate 202 coplanar with the
conductors 206 and 210. The conductor 230 has one inner edge 232
spaced away from and facing another edge 234 of the first coplanar
conductor 206, forming a gap 233. The edges 232 and 234 form
coplanar edges for propagating RF signals of wavelength
.lambda..sub.s along with edges 208, 212 as part of CPW or coupled
slotline wave guide 231.
The conductor 230 has an outer edge 236 spaced a distance Wc2' away
from inner edge 232 to form another part of a coplanar waveguide
structure associated with conductor 210, gap 214 and conductor 206.
A second lossy resistive film 238 has a first edge 240 conductively
coincident with the outer edge 236 of conductor 230. An outer edge
242 of the film 238 is spaced away from first edge 240 by a
distance Wr'.
Structure 231 is found to be effective in attenuating spurious MSM
propagation over the desired frequency band without significantly
attenuating the desired CPW mode signals. For alternative CPW mode
suppression transmission lines having a longitudinal dimension of
many wavelengths, the width, Wr, may be significantly less than
1/4.lambda..sub.x and still be effective. It is anticipated that a
mode suppression transmission line in accordance with this
invention will require a greater length along the signal conductors
206 and 210 for a given amount of attenuation, as the width, Wr, is
decreased.
With reference to FIG. 2, films 220, 238 may each be a "sea of
resistor", implying that the resistive films may extend over large
areas of the substrate 204 not otherwise covered with circuit
patterns. Covering extensive areas of the substrate 204 with the
extension of the resistive films 220, 238 further enhances
attenuation and suppression of spurious modes and comes at no
additional cost in the process of manufacturing the coated and
patterned substrate 204. This is particularly true for
manufacturing processes which already utilize coplanar resistive
films to define discrete or distributed resistors, for example, as
terminations, attenuators, bias elements or the like.
With regard again to FIG. 2, in other embodiments of the invention,
the edges 222 and 240 of the resistive films do not have to be
coincident with the respective conductor edges 216 and 236, but
instead may be spaced apart (not shown). This is indicated by
alternative edges 222' and 240' (shown as dashed lines) of the
resistive film 220 and 238 spaced apart from the conductive edges
216 and 236 by gaps 237 and 241 such that the resistive films 220
and 238 are conductively separated but electromagnetically coupled
to conductors 210 and 230. The separation of edges 216, 222' and
236, 240' creates additional fields within the gaps 237 and 241
which may couple additional energy from undesired spurious modes
into the resistive films 220, 238.
SUPPRESSION OF MSM WAVES
Referring to FIG. 3 in comparison with FIG. 11, there is shown the
current distribution, Jc and Jc", (taken along the line 3--3 of the
coplanar mode suppression resistive structure 231 shown in FIG. 2)
compared with the current distribution for the prior art CPW
structures. The maximum of the current density, Jc", of MSM at the
outer edges of the conductor strips 210 and 230 is attenuated
(compared to the magnitude of Jc' in FIG. 11) by the influence of
the current density, Jc", in the facing edges of the resistive
strips 220 and 238 without appreciably affecting the current
distribution, Jc, of the CPW mode in the center conductor strip 206
and the inner edges 212 and 232 of the strips 210 and 230.
In a preferred embodiment of this invention, the resistive films
220 and 238 may be separated from the respective adjacent conductor
edges 216 and 236 by respective gaps 237 and 241. The width of gaps
237 and 241 are typically as small as possible for a given
substrate fabrication process. The gaps 237 and 241 may provide
additional attenuation of the spurious mode due to MSM surface
currents at the edges 222, 240.
Wr and Wr' are selected to provide significant loading to the MSM
electromagnetic fields which would otherwise propagate along the
substrate 202. With reference to FIG. 4, there is shown an
elevation view of a cross section of coplanar transmission line 231
of FIG. 2 taken along the line 3--3, in which the transmission line
231 is disposed within a cavity housing 248. The effectiveness of
significant loading to the MSM electromagnetic fields can be seen
with the aid of the directed arrows, Em. The arrows, Em, represent
the electric field distribution of undesired MSM that are supported
by the CPW conductors 206, 210 and 230 as a whole. Undesired MSM
have a component of electric field, Em, which is tangential (not
shown) to the substrate surface.
Returning again to FIG. 2, the resistive films 220 and 238 of
sufficient width (Wr and Wr') attenuate currents associated with
MSM waves which are flowing at or near the coincident edges 216,
222 and 236, 240. Also, components of electric field tangential to
the films 220, 238 will be attenuated. Longer wavelength MSM waves
will be even more effectively attenuated by the films 220 and 238
shown in FIG. 2, if the width Wr and Wr' are on the order of
1/4.lambda..sub.x or larger, where .lambda..sub.x is the wavelength
of the spurious mode of concern. Larger dimensions of Wr and Wr'
are increasingly effective for lower frequencies.
In the limit as Wr and Wr' become very large, attenuation of MSM
waves is effective to very low frequencies, well away from the
bandwidth of interest. Also as Wr and Wr' become large, the films
220, 238 appear as discrete resistors for very low frequencies and
can be conveniently connected to circuit ground points (not shown)
in locations which do not affect the RF signals propagating along
the CPW lines 206, 210, 230.
In FIG. 4, upper and lower oppositely directed arrows Em represent
electric field vectors of the MSM. Higher order modes, such as slab
modes, will be even more effectively suppressed since their
wavelengths are even shorter than those of the fundamental mode
associated with the overall dimension Wa and the widths, Wr, Wr' of
the resistive films are a greater multiple of .lambda..sub.x /4,
the spurious mode quarter-wavelength.
SUPPRESSION OF WAVEGUIDE OR CAVITY MODES
With reference to FIG. 4, there is shown an embodiment of waveguide
or cavity mode suppression of coplanar microwave circuits in
accordance with the present invention. The CPW transmission line
231 is enclosed in a conductive housing 248 having a height Hw and
a width Wa. The housing 248 is thus a waveguide or cavity and can
support undesired electromagnetic waveguide or cavity mode waves
above cutoff, e.g. .lambda.x<2Wa. The dashed arrows, Ew,
represent electric fields of waveguide or cavity modes in the
housing 248. In the present invention, the resistive films 220 and
238 intercept the electric or magnetic field lines of the waveguide
or cavity modes, thereby suppressing and attenuating them. The
widths, Wr, Wr' of the films 220 and 238 are selected to be greater
than a minimum value for suppressing undesired waveguide, slab or
cavity modes. A resistive film 220, 238 having a width, Wr, Wr',
greater than or equal to about .lambda..sub.x /4, provides
sufficient attenuation per unit length of waveguide or cavity modes
to effectively suppress such modes from propagating within the
waveguide or cavity 248.
Referring again to FIG. 2, although not shown, the edges 236 and
240 may alternatively be overlapped, with an insulating layer
therebetween, to achieve conductive isolation, but with
electromagnetic coupling between the conductor 230 and film
238.
In general, the attenuation per unit length of the resistive film
structure (SOR) of this invention will depend on the mode and
wavelength of the signal to be suppressed, the sheet resistance of
the resistive film, the width of the film and the particular
configuration of the facing edges of the resistive film and the
corresponding adjacent coplanar conductive strip. The total
attenuation to achieve the required suppression of unwanted modes
in a specific circuit will depend on the gain of the active
elements included in the circuit and the layout of the circuit.
With reference to FIG. 5, there is shown an alternative embodiment
of the present invention for waveguide or cavity mode suppression
extended from that of FIG. 2. In FIG. 5 the same elements are
numbered with the same reference numerals as in FIG. 2. The CPW
mode suppression structure of FIG. 5 is provided with coplanar
resistive films 220' and 238' in which films 220' and 238' take the
place of films 220 and 238 in FIG. 2. The films 220' and 238' are
not a solid sheet of uniform resistivity, as before shown in FIG.
2.
With reference to FIGS. 5 and 6 the films 220' and 238' are
patterned to form a mesh or lattice work 250 formed from a
resistive film having intrinsic sheet resistivity, Rs. The
patterned mesh 250 may have a predetermined ratio of open,
insulating area to covered, resistive area, R.sub.oc. The resistive
films 220' and 238' may also be defined as a sheet having a
plurality of apertures 252 therein. The apertures 252 may be
arranged in a regular array, e.g., forming the mesh 250 of
resistive material.
With appropriate patterning of a resistive film as a periodic mesh,
the resistive film having low intrinsic resistivity can effectively
function as a sheet of higher average resistivity for nodes having
wavelengths of dimension greater than several multiples of the
pattern period. This provides a method for varying the average
sheet resistance at different points on the circuit, without
changing the intrinsic sheet resistance of the resistive film.
The mesh 250 is characterized by a lateral dimension Wm, forming
essentially zero conductivity apertures 252 of dimension Wo. The
dimensions Wo and Wm of the lattice work 250 and the openings 252
are selected such that the average sheet resistivity, Ravg, of the
films 220' and 238' over several multiples of the repetitive
pattern period, Wm+Wo, is higher than the intrinsic sheet
resistance, Rs, of the material 250. The mesh 250 may be formed by
etching a continuous sheet through an etch resist, by punching or
by an overlay of crossing resistive strips as is well known.
Typically, the resist mesh 250 would be etched from a deposited
film, such as nichrome, tantalum nitride or the like deposited on
the substrate surface 204. The mesh 250 may be formed from a
resistive sheet having the open apertures 252, to form a grid, or
formed by holes spaced at suitable intervals etched or punched in a
sheet bonded to the substrate.
A preferred intrinsic sheet resistance, Rs, for a deposited
resistive film 250 is about 50 ohms/square. Films 220' and 238' can
be patterned with the ratio of the area of the apertures 252 to the
area of the mesh 250 to yield a higher average sheet resistance to
provide a dose match to the characteristic impedance of waveguide
or cavity modes that are incident on the film 250. The mesh
configuration of a resistive film in accordance with this invention
is effective in suppressing both spurious microstrip, waveguide,
slab and/or cavity modes.
The apertures 252 may also be arranged in an aperiodic, irregular,
or semi-random arrangement (not shown), as desired, to present
different average sheet resistances in different areas. An
aperiodic, semi-random pattern could be used to attenuate a range
of undesired modes having a range of characteristic impedances over
a broad range of frequencies.
In areas that require an average sheet resistance lower than the
intrinsic sheet resistance of the resistor material, the mesh can
be formed by overlaying the intrinsic sheet with a conductor
pattern, in which the former apertures 252 are replaced with
conductor material, and the stripes 250 are made of a resistive
film. The relative size of the apertures, Wo, and the stripes, Wm,
are adjusted to achieve the desired average sheet resistance.
With reference to FIG. 7, there is shown a partially cut away
perspective view of an exemplary embodiment 300 of several CPW
spurious mode suppression circuit structures in accordance with
this invention. The structure 300 is mounted on an insulating
planar substrate surface 301 disposed in a housing 302. The
structure 300 includes a sequential series of flip-chip mounted,
mm-wave, active components 304, 306, 308, mounted on the substrate
planar surface 301. The active component structures 304, 306, 308,
may include flip-chip amplifiers, oscillators, filters and the
like. Each of the components has one or more flip-chip mounted
input connections and one or more flip-chip mounted output
connections. The components 304, 306, and 308 may also include
multi-component sub-assemblies of hybrid or integrated circuit
type.
The component structures 304, 306, 308, are connected in series
from respective flip-chip mounted output connections to respective
flip-chip mounted input connections (not shown) by opposed ends of
respective mode suppression CPW transmission line segments 314,
316, 318. The mode suppression CPW transmission line segments 314,
316, 318 are defined on the substrate 301 in accordance with the
present invention. The CPW transmission line segments 314, 316, 318
are constructed according to the mode suppression principles
discussed above with reference to FIGS. 1-6.
With reference to FIG. 8, a magnified view of a flip-chip mounted
active device 306 inset in FIG. 7 is shown connected between
longitudinal signal input CPW transmission line 314 and
longitudinal output CPW transmission line 316. Input CPW
transmission line 314 includes segment 314e centered between
parallel ground strips 314c and 314d. Output CPW line 316 includes
signal segment 316a centered between coplanar ground strips 316c
and 316d.
Coplanar ground strips 314c and 314d are spaced apart from outer
edges of conductor 314e by insulating gaps of width Wg. Ground
strips 314c and 314d are spaced apart from and electromagnetically
coupled to laterally opposed coplanar resistive films 330 and 332.
The films 330 and 332 define respective resistive film edges 330a,
and 332a, spaced apart from respective outer edges of ground
conductors 314c and 314d by gaps 314a and 314b of width Wg2.
Undesired spurious modes along the conductors 314e, 314c and 314d
are suppressed by the coupling between the ground strips 314c, 314d
and the adjacent resistive films 330, 332.
The resistive film edges 330a and 332a extend proximally along the
conductors 314c and 314d. The resistive film edges 330a and 332a
are offset laterally inward proximal to the input 306a to form
edges 330b and 332b conductively coupled with respective outer
edges of coplanar ground conductor strips 316c and 316d. Resistive
film edge 330b extends conductively coincident with an outer edge
of conductor edge 316c to the next device 308. Resistive film edge
332b' extends conductively coincident with an outer edge of
conductor 316d' to the next device 308.
Resistive film edge 332b extends conductively coincident with the
conductor edge 316d to a T-junction formed by ground conductors
316c, 316d and signal conductor 316a with ground conductor 316d'
and signal conductor 316a'. Conductors 316c, 316a and 316a are the
output of device 306. Conductors 316c, 316a, and 316d' are the
input of device 308. Conductors 316d, 316a' and 316d' are the CPW
connection to the element 336 (of FIG. 7). The element 336 may be a
tuning element or a bias input element as described below.
Undesired spurious modes along the conductors 316a, c, d, a' and d'
are suppressed by the conductive coupling between ground conductors
316c, d, d' and the respective resistive film edges 330b, 332b,
b'.
The flip-chip 306 has input flip-chip bump 306a mounted on the
input line 314e, output flip-chip bump 306b mounted to output line
316a and laterally opposed common ground, flip-chip bumps 306c and
306d mounted to joined ends of the respective ground strips 314c,
316c and 314d, 316d.
A coplanar ground conductor 317 disposed between the input 306a and
output 306b connects the joined ends of 314c, 316c to the joined
ends of conductors 314d, 316d.
The CPW segment 318 feeds one input port of a mixer 324. A second
set of series components 310, 312 mounted on the substrate are
connected by opposed ends of CPW transmission line segments 320,
322 respectively, defined on the substrate 301, to another input
port of the mixer 324. The CPW transmission line segments 320, 322
are defined on the substrate 301 in accordance with the present
invention. The mixer 324 inputs from segments 318 and 320 are mixed
and fed from the mixer 324, to an amplifier 326 through mode
suppression CPW segment 328. The amplifier 326 is connected to an
output (not shown) through a final mode suppression CPW coplanar
transmission line structure 334.
Passive bias and matching circuitry elements (not shown) may also
be connected to the CPW segments 314-318 with similar mode
suppression topologies.
Coplanar resistive films 330 and 332 are laterally disposed on
opposite sides of the CPW transmission line segments 314, 316, 318,
320, 322. The films 330, 332 extend laterally outward on the
substrate 301 to a width sufficient to suppress microstrip modes
and waveguide or cavity modes otherwise present in the structure
300. The coplanar resistive films 330 and 332 may extend
transversely to form SOR absorbing films extending over otherwise
unused portions of substrate 301.
The structure 300 may be enclosed in a simple rectangular housing
302 characterized by a lateral width dimension, W, a length
dimension, Lh, and a height dimension, Hw1'. The dimensions Lh, W
and Hw1' of the housing 302, and the substrate 301 contained
therein, are not restricted to be tightly controlled to suppress
microstrip and waveguide or cavity modes. The simple rectangular
geometry of the substrate 301 and housing 302 is less expensive and
more robust than previous art substrates and housings which must
closely follow the CPW circuit topology and which must be tightly
controlled to suppress spurious waveguide or cavity modes.
Different areas of the circuit structure 300 may have differing SOR
structures depending on the desired mode suppression structure. As
described above, the resistive films 330 and 332 adjacent to the
CPW segment 314, may be spaced apart from the opposed sides of the
segment 314 by a respective parallel gap 314a and 314b
therebetween. In contrast, the films 330 and 332 adjacent to the
CPW segments 316, 318, 320 and 322 are contiguous with the
respective opposed sides of the respective segments and have a
lateral width, Wr, sufficient to attenuate the undesired modes.
The gaps 314a and 314b have a width, Wg2, adjusted to provide the
desired degree of electromagnetic coupling between the films 330,
332 and the conductive segments 314c, d. At a frequency range of
about 20 to 40 GHz, the gap width, Wg2, may be from about 0.025 mm
to about 0.05 mm (1-2 mils) and may extend along the segments for
many wavelengths.
The element 336 is surrounded by coplanar resistive film or mesh
338 and 340, defining an edge 342. The film 338 is an extension of
the film 332.
In an instance where lower loss is desired, the edge 342 is spaced
laterally away from the element 336 by an insulating aperture 344
defined in the mesh 338, 340. The edge 342 is spaced sufficiently
far from the element 336 to prevent appreciably attenuating the
desired signal by the mesh 338.
The material of the mesh films 338, 340 has intrinsic sheet
resistivity the same as the material of the films 330, 332, but may
be patterned at a process step to provide a mesh composed of
stripes 250 and apertures 252 (shown in FIG. 6). The mesh films
338, 340 may be defined as before with stripe 250 width and
aperture 252 dimensions to present the desired average sheet
resistance in the mesh areas, suitable for attenuating undesired
waveguide or cavity, slab and surface wave modes. Different stripe
250 width and aperture 252 dimensions may be selected to provide
different average sheet resistances in different circuit areas by
merely incorporating the different patterns in the same patterning
process step.
Embodiments of this invention incorporating circuits with CSL
connections can also be constructed. With reference to FIGS. 7 and
8, a CSL structure may be constructed by omitting a portion of the
ground conductor 314d between active device 304 and device 306,
omitting the ground conductor 316c between the device 306 and
device 308 and repositioning the resistive film edges 330a, b and
332a, b. In a CSL embodiment of the present invention, the
resistive film edges 330a and 332a are repositioned such that they
are spaced equally away from the respective outer edges of 314c and
314e.
The resistive film edges 330b and 332b are repositioned such that
they are essentially contiguous with the respective outer edges of
conductors 316a and 316d. In this case, the input to device 306 is
connected to a CSL consisting of facing edges of conductors 314e
and 314c. The output of device 306 is connected to a CSL consisting
of facing edges of conductors 316a and 316d. The repositioned
resistive film edges 330a, b and 332a, b suppress unwanted spurious
modes by coupling to respective outer edges of conductors 314c,
316e and 316a, 316d.
Continuous coplanar resistive films extending between an input and
an output of active devices, may introduce undesired coupling
between the input and output. An additional spurious mode
suppressing feature of the combined active device, coplanar
resistive film connections of this invention is achieved with
resistive film decoupling slots. One example of the resistive film
decoupling slot aspect of the present invention is shown with
regard to device 326 and components 304, 306 and 308 of FIG. 7.
The films 338 and 340 may be formed to define a decoupling slot 346
interposed between the device 326 and the components 304, 306, and
308. The slot 346 is configured to provide an insulating electrical
discontinuity or gap between the films 338 and 340 along the
surface 301. The configuration of the electrical discontinuity of
the slot 346 is arranged to essentially inhibit or suppress
transmission or communication of undesired spurious signals through
the resistive films 338 and 340 between the respective output of
device 326 and the respective inputs of components 304, 306, and
308.
The mesh films 338 and 340 may be completely separated by the
de-coupling slot or aperture 346 extending between two opposed open
ends 346a and 346b. The end 346a is disposed on one side of the two
devices 306 and 326. The opposite end 346b is disposed on the other
side of the two devices 306 and 326. The decoupling slot 346
between the resistive mesh films 338, 340 provides additional
suppression for undesired signals which might otherwise couple from
the output of amplifier 326 through the films 338, 340 into an
input of one of the components 304, 306 and 308. The slot 346 may
be configured as a rectilinear aperture, a semi-circular arc
segment, a meandering, or serpentine aperture of uniform or
tapering width or an irregular aperture as desired.
The film 347 defines an additional decoupling slot 348 therein
having a length between an open end 348a and a closed end 348b. The
decoupling slot 348 is interposed between the inputs of component
310, 312 and the output of amplifier 326. The position and length
of the slot 348 in the film 347 is adapted such that undesired
signal modes in the film 347 are essentially decoupled between the
inputs of components 310, 312 and the output of amplifier 326. The
open end 348a terminates at one edge of the film 347 at one side of
components 310, 312 and 326. The closed end 348b terminates within
the resistive film 347 near an opposed side of the group of
components 310, 312 and 326.
The shape, size and location of a resistive film decoupling slot
for a specific circuit topology may be determined empirically,
i.e., by scratching, scoring, cutting, grinding, grooving, or
otherwise abrading an aperture in the film of an actual circuit.
Alternatively, simulations may be performed to determine the extent
to which a decoupling slot is required for suppressing unwanted
coupling from one portion of the circuit to another.
Additional decoupling slots may be interposed at different
locations on the mode suppression films. Another coplanar mode
suppression resistive film 347 is disposed on the substrate 301
between the components 310, 312 and the amplifier 326. The film 347
suppresses propagation of undesired modes along CPW transmission
lines 320 and 322.
The decoupling slots 346, 348 may be formed by standard patterning
and etching processes or may be formed by abrading, cutting,
scratching or otherwise removing a portion of the respective films
338, 340, 347.
Another mode suppression structure in accordance with this
invention is also shown with regard to FIG. 7. A lossy bias CPW
resistive film mode suppression structure 350 is disposed on the
substrate 301. The lossy bias structure 350 includes a longitudinal
bias conductor stripe 356 having opposed sides and opposed ends
formed on the substrate 301. Two bonding pads 352, 354 are
connected at respective end of bias conductor stripe 356. At least
one connection 352, 354 is configured for connection to a flip-chip
active device (e.g., device 326) by a wire bond or coplanar
connection (not shown). The other connection is configured for
connection to an external power supply.
On opposed sides of the conductor strip 356 there are no laterally
opposed, longitudinally extending coplanar resistive film strips
358 and 360 parallel to and contiguously adjoining the respective
sides of the conductor stripe 356. The resistive strips 358 and 360
extend generally parallel to and longitudinally along the conductor
strip 356 for a distance, Lrb, and extend laterally away from the
conductor stripe for a distance, Wrb.
A second pair of laterally opposed resistive strips 362, 364 having
predetermined length and width and opposed inner and outer edges
are disposed on the substrate 301. The inner of resistive strips
362, 364 edges are parallel to and spaced away from the resistive
strips 358, 360 by respective insulating gaps 366, 368 having a
width, Wgb. Two laterally opposed ground conductors 370, 372, are
connected to the respective opposed outer edges of the second
resistive strips 362, 364.
Bias, such as a DC voltage, may be connected (not shown) to pad
352, for connection by wire bonds or other means from pad 354 to
one or more of the components of the circuit 300. The bias
conductor 356 provides a low DC resistance for efficient transfer
of DC or low frequency power from an external supply (not shown) to
the circuit components while absorbing or attenuating the
transmission of high frequency signals between the ends 352, 354 of
the conductor strip 356. The length, Lrb and widths, Wrb, Wgb, of
the respective conductor strip 356, resistive strips 358-364 and
gaps 366, 368 are selected to provide the desired attenuation of
mm-wave signals travelling from one portion of the coplanar circuit
to another.
Additional series and shunt elements, such as chokes and filter
capacitors (not shown) may be added as is well known by those
skilled in the art, for additional filtering. The advantage of the
lossy bias structure 350 is that it is easily formed from the
processes used to construct the other CPW components and resistive
mode attenuation structures of this invention.
The bias conductor strip 356 and adjacent resistive strips 358, 360
may be formed into coplanar configurations other than that of a
simple straight line having a uniform cross-section. The conductor
strip 356 may be tapered, uniformly or logarithmically, or may be
segmented into multiple rectilinear sub-sections joined end to end
having the resistive strips 358, 360 suitably aligned alongside.
The strip 356 may also be formed as a longitudinal conductor having
one of number of shapes; e.g. a monotonically increasing linear
taper, a logarithmic or exponential taper. The strip 356 may also
be curvilinear, e.g., a coplanar uniform width spiral or tapered
spiral or the like, connected to or coupled with a plurality of
adjacent resistive coplanar film elements.
In a monolithic semiconductor integrated circuit embodiment of the
present invention, having the same surface topology as shown in
FIG. 7, it is contemplated that the planar surface 301 is the
surface of a monolithic bulk semiconductor 380, such as GaAs,
having the active (and/or passive) devices 304, 306, 308, 324, 312,
310, 326 defined therein by integrated circuit techniques; e.g.,
film deposition, photolithographic definition, implantation, and
diffusion process steps. The conductors 314c, d, e and 316a, c, d,
and the resistive films 330, 332 are defined and connected to the
active devices at a process step included in the integrated circuit
fabrication process.
Other insulating and semi-insulating substrates may be used in
alternative embodiments of the present invention. Materials such as
semi-insulating GaAs, indium phosphide, high resistivity silicon,
diamond and silicon carbide are contemplated as substrates for
flip-chip embodiments of this invention. These same materials are
also contemplated as suitable for monolithic integrated circuit
embodiments having the active devices configured therein.
With regard to FIG. 13, there is shown an interdigitated resistive
coplanar mode suppression structure 400 in accordance with this
invention connected to a coplanar flip-chip mounted active device
402. The coplanar, interdigitated resistor, mode suppression
structure 400 and the flip-chip 402 are mounted on an insulating
substrate surface 404. The structure 400 is configured to connect a
low frequency signal or slowly varying bias, Si, from an input
device (not shown) along a low resistance conductor strip 406 from
one end 406a to a second end 406b. The conductor strip 406 may be a
signal or bias input, for example, used to activate a mm-wave
component. The second end 406b is connected to an input 408 of the
flip-chip 402 through a component 410 and connection 410a. The
component 410 may be a coplanar resistor formed as part of the
resistive structure herein described, or may be a lumped component
such as a flip-chip resistor. The connection 410a may be a coplanar
connection, a wire bond, or the like connected to a coplanar
pattern 411 for receiving the flip-chip bump 440 connection to
input 408.
The signal conductor 406 is configured as a longitudinal strip
extending for a length, Lrb, between the two ends 406a and 406b.
Disposed on either side of the strip 406 are first and second
opposed coplanar resistive structures 412 and 414 having a
respective plurality of spaced apart, coplanar comb-like resistive
film fingers 412a and 414a. The resistive film fingers 412a and
414a each have an inward facing end proximal to the respective
opposed sides of the conductor strip 406. The other end of each of
the respective resistive film fingers 412a and 414a are distally
disposed, outward facing from the respective sides of the strip
406.
A first and second plurality of coplanar resistive strips 416 are
disposed crossing the conductor strip 406. The resistive strips 416
have opposed distal ends projecting orthogonally away from either
side of the strip 406. The second resistive strips 416 are arranged
such that each strip 416 has one distal end disposed between and
partially interdigitated with a pair of inward facing, adjacent
resistive film fingers 412a. The other distal end of each strip 416
is disposed between and partially interdigitated with a
corresponding pair of oppositely aligned, inward facing, adjacent
resistive film fingers 414a. The strips 416 and the fingers 412a
and 414a are spaced apart by a width, Wgb, forming respective
insulating meandering gaps, 418, 420 therebetween.
A pair of coplanar conductive ground strips 426, 428 are positioned
on the substrate 404 distally adjacent to and electrically coupled
with respective opposite distal ends of resistive film fingers 412a
and 414a. The strips 426 and 428 are preferably conductively
connected with the distal ends of strips 412a and 414a, but may be
alternatively spaced apart and electromagnetically coupled.
One end of the coplanar ground strips 426, 428 is located adjacent
to device 402. Coplanar electrical connections 430, 432 are
provided between adjacent ends of coplanar conductors 426, 428 and
respective flip-chip ground terminals 434, 436 of the device 402. A
coplanar common ground strap 435 positioned between input and
output terminals 408, 438 connects the common ground terminals 434,
436. A coplanar output connection 439 connects output terminal 438
to an output structure (not shown).
The resistive film fingers 412a, 414a, and the strips 416 are
separated by gaps 418, 420. Fingers 412a and strips 416 may
therefore also be described as being disposed on longitudinally
disposed lengths of conductors 426 and 406. The signal or bias
conductor 406 and the ground conductors 426, 428 in combination
with the resistive strips 414, 412 and 416, form a lossy
transmission line effective in attenuating mm-wave signals
propagating along the conductor strip 406 without significantly
attenuating DC power, or sufficiently low frequency signals,
conducted along the conductor strip 406.
The resistive strips 416 and resistive film fingers 412a, 414a are
dimensioned with respective lengths, Ls, Lf, and widths Ws, Wf, to
achieve a desired attenuation at a specific frequency. The
attenuation of the typical coplanar attenuation structure 400 in
accordance with this invention having a resistive film of about 50
ohm/square resistivity is shown in the accompanying Table I with
the referenced dimensions.
The data of Table I was obtained from a structure 400 having gold
conductors and 50 ohms per square resistive film, mounted on a BeO
substrate about 0.63 mm (25 mils) thick. For comparison, a straight
line resistive structure 350 of the form shown in FIG. 7 of the
same length and comparable width and spacing dimensions is less
effective for the same circuit area.
TABLE I ______________________________________ Ls = 8 mil, Lf = 7
mil, Wgb = 1 mil, Ws = Wf = 1 mil, Lrb = 260 mil Loss, S21 dB Loss,
S21 dB Frequency, GHz FIG. 7 FIG. 13
______________________________________ 10 2 8 20 6 22 30 12 >39
40 19 >38 50 28 >38
______________________________________
Alternatively, the resistor structure 400 and the flip-chip 402
could be on separate substrates, instead of a single substrate 404.
Lossy CPW mode suppression structures 400 may be used as input
lines on microprobe structures for probing high frequency devices.
The connections 410, 430, 432 from the structure 400 to the device
408 may be made by removably connecting probes, with connections
410, 430 and 432 provided by probes instead of coplanar conductors
on the same substrate. The structure 400 could also be used to
provide DC power or bias voltage to an active device in a mm-wave
coplanar circuit.
With regard to FIG. 14, a cross section of a preferred coplanar
configuration of the mode suppression structure 400 in combination
with a flip-chip active device is shown taken along the line 14--14
of FIG. 13. The conductors 426, 428, terminals 408, 434, 436, and
resistive strips 412a, 414a, 416 are formed as coplanar elements
along the surface 404. The flip-chip mounting of the active device
402 is shown by flip-chip bumps 440 connecting between the
respective conductive patterns (not shown) on the device 402 and
the terminals 408, 434, 436.
With respect to FIGS. 15 and 16 there is illustrated an alternative
lossy bias resistor structure 500, for supplying bias to mm-wave
circuits while absorbing spurious mm-wave signals in accordance
with this invention. FIG. 16 is a schematic equivalent circuit of
the signal absorbing resistor structure 500.
Two longitudinally disposed, coplanar ground conductor strips 502
and 504 of width, Wg, are defined on the substrate 404. The strips
502 and 504 are parallel to and spaced apart from each other, with
opposed proximal and distal ends 502a, b and 504a, b and inward
facing edges 502c and 504c. One end of each strip 502a, 504a, is
connected to the respective device ground conductors 316d and 316d'
(also shown in FIG. 8). The other ends 502a, 504b, extend distally
away from the ground conductors 316d and 316d'.
A coplanar signal conductor strip indicated generally by the
numeral 506 is defined on the substrate 404 between the inside
edges 502c and 504c. The signal strip 506 is configured as a
meandering slow wave transmission line composed of a continuously
connected succession of coplanar conductive segments. The strip 506
includes a first plurality of longitudinal conductive segments 508,
and second plurality of lateral conductive segments 510. Each of
the segments 508 and 510 are of equal width, Ws1 between opposed
sides. The signal strip 506 extends a distance, Lrb, between the
proximal end 506a connected to the coplanar signal conductor 316a'
and a distal end 506b located near the ground conductor distal ends
502b, 504b. The proximal and distal ends 506a and 506b are located
sufficiently near the respective proximal and distal ends of the
two ground conductors to present a suitable impedance and
transmission characteristic for the coplanar line 506 as is well
understood in the art. The combination of each pair of adjacent
lateral segments 510 and the adjoining longitudinal segment 508 is
referred to as forming a U-shaped loop open laterally toward an
associated ground conductor strip 502 or 504.
The second plurality of lateral segments 510 are each of equal
length, Lc2, between first ends and laterally opposed second ends
and are positioned orthogonally between the ground conductors 502,
504. The first ends and opposed ends of segments 510 are spaced
away from the respective inner edges 502c, 504c of the respective
ground conductors 502, 504 by a gap spacing, Dg.
A first one of the segments 510 is located proximally to the
proximal strip end 506a. A second one of the segments 510 is
displaced distally from the first one of the segments 510 by the
length, Lc1, of the segments 508. Each successive segment 510 is
displaced distally from the previous segment 510 by the length,
Lc1, of the segments 508, forming alternate pairs of equally spaced
segments 510 with respective adjacent first ends and opposed
ends.
The first plurality of longitudinal segments 508 are each of equal
length, Lc1, between respective proximal and distal ends and are
oriented parallel to the ground conductors 502, 504. A first one of
the longitudinal segments 508 is disposed between the first and
second lateral segment 510. The first one of the longitudinal
segments 508 has the proximal end joined with the first end of the
first lateral segment 510 and the distal end joined with the
adjacent first end of the second lateral segment 510. A second one
of the longitudinal segments 508 has the proximal end joined with
the opposed end of the second lateral segment 510 and the distal
end joined with the adjacent opposed end of the third lateral
segment 510.
Each succeeding longitudinal segment 508 has the proximal end
joined with the lateral end of the preceding lateral segment which
is opposite to the lateral end joined to the distal end of the
preceding longitudinal segment. The distal end of the succeeding
longitudinal segment 508 is joined with the lateral adjacent end of
the succeeding lateral segment 510. Each succeeding longitudinal
segment is thus adjacent to opposite conductor ground strips 502,
504. Each longitudinal segment 508 has an outer edge spaced away
from the corresponding ground strip by the gap distance, Dg.
Mm-wave signals introduced at the distal end 506b are absorbed
along the structure 500 toward the proximal end 506a by attenuation
from the coplanar resistive films coupled with the signal line 506,
as described below. Conversely, mm-wave signals from the conductor
316a' will be absorbed along the line 506 toward the distal end
506b. This prevents coupling of RF signals into or from the power
supply and subsequently from or into other active circuits on the
common substrate 404.
A first plurality of coplanar lateral resistor strips 516 is
defined on the substrate 404. Each resistor strip 516 has a lateral
length, Y1, between an inward facing end 516a and an opposed
outward facing end 516b, the length being orthogonal to the ground
conductors 502, 504. Each lateral resistor strip 516 has a
longitudinal width, X1, between opposed lateral sides connecting
the opposed inward and outward facing ends. Each successive strip
516 has the respective inward and outward ends oriented oppositely
to the inward and outward ends of the preceding and following
strips 516.
A first one of the resistor strips 516 is disposed between the
first and second adjacent lateral conductor segments 510. Each side
of the strip 516 is spaced away from the respective segments 510 by
the gap distance, Dg. The inward facing end of the strip 516 is
directed toward the corresponding first longitudinal segment 508
disposed between the first and second adjacent lateral segments
510. The outward facing end of the first resistor strip 516 is
connected to the inner edge of the ground conductor 504 opposite to
the connecting first longitudinal segment 508. Each resistor strip
516 forms a lossy ground conductor along either side, adjacent to a
first portion 512 of each of the adjacent lateral segments 510. In
a conventional slow wave transmission line structure, the resistor
strip 516 would be a high conductivity conductor.
A second one of the resistor strips 516 is disposed between the
second and third adjacent lateral conductor segments 510. Each side
of the second strip 516 is spaced away from the respective segments
510 by the gap distance, Dg. The inward facing end of the second
strip 516 is directed toward the corresponding second longitudinal
segment 508 connected between the opposite ends of the second and
third adjacent lateral segments 510. The outward facing end of the
first resistor strip 516 is connected to the inner edge of the
ground conductor 502 opposite to the connecting second longitudinal
segment 508. Each resistor strip 516 forms a lossy ground conductor
along either side, adjacent to a corresponding first portion 512 of
each of the second and third adjacent lateral segments 510.
Each succeeding one of the lateral resistor strips 516 is similarly
disposed between and spaced from the corresponding preceding and
succeeding adjacent lateral strips 510. Each side of each
succeeding resistor strip 516 is spaced away from the respective
preceding and succeeding lateral segments 510 by the gap distance,
Dg. The inward facing end of each succeeding strip 516 is directed
toward the corresponding longitudinal segment 508 connected between
the opposite ends of the corresponding preceding and succeeding
adjacent lateral segments 510. The outward facing end of each
succeeding resistor strip 516 is connected to the inner edge of the
ground conductor opposite to the corresponding connecting
longitudinal segment 508. Each succeeding resistor strip 516 forms
a lossy ground conductor along either side, adjacent to a
corresponding first portion 512 of each of the preceding and
succeeding adjacent lateral segments 510.
A second plurality of coplanar resistor strips 518 is defined on
the substrate 404. Each resistor 518 has a periphery having a first
U-shaped portion 518b coincident With the inner edge of each
respective longitudinal strip 508 and a portion 514 of each of the
respective connected adjacent lateral strips 510. The balance of
the periphery 518a forms a longitudinal edge facing the inward
facing end of the respective resistor strip 516. Each resistor
strip 518 and laterally adjacent resistor strip 516 are accordingly
referred to as being disposed on laterally adjacent lengths of
conductor 510 and respective ground conductor 502 or 504. The
longitudinal edge 518a is spaced from the respective inward facing
end of 516 by the gap distance, Dg. Each resistor 518 defines a
lateral length, Y2, between the inner edge of the respective
longitudinal conductor strip 508 and the respective peripheral edge
518a along the portions 514 of the adjacent lateral strips 510.
The overall lateral size of the bias structure 500 is:
TABLE II ______________________________________ Lrb 20 mils Wg 4
mil Dg 1 mil Ws1 1 mil X1 1 mil Y1 11 mil X2 3 mil Y2 8 mil
______________________________________
The overall longitudinal size of the bias structure 500 is Lrb, not
including connections to the opposed ends 506a and 506b.
In alternative embodiments of the structure 500, the widths of the
segments 508 and 510, and the lengths of segments 508 and 510 may
be unequal. The position of the segments 508 and 510 between the
ground conductors 502 and 504 may also be nonuniform. For example,
the segments position, length and width may vary as a log periodic
function along the length of the conductors 506, 502 and 504. It is
also contemplated that the topology of the coplanar structure 500
may be curvilinear, e.g., a spiral or semicircle or the like along
the length of the conductors 506, 502 and 504.
FIG. 16 depicts an equivalent circuit schematic of the meandering
lossy bias structure 500. The inductive component of ground
connections 502, 504 are represented as lumped inductors L2. The
resistive film strips 516 are represented as resistors Rf.
Capacitors, C, represent the capacitive coupling between sides of
the resistive film strips 516 and the adjacent lateral portions 512
of lateral segments 510. Resistor Rs represents the resistive
component of the resistive film strip 518 coupled to the inductive
component, L1, contributed by the series connection of the
longitudinal segment 508 and adjacent portions 514 of lateral
segments 510, and in series with the inductive component, L3,
contributed by the portions of lateral segments 510 located between
adjacent portions 514.
Both resistors are important for optimum absorption. If resistive
film strip 518 is too long, the resistive film strip 516 will be
too short to act as lossy grounds for the strip transmission line
segments 510. If resistive film strip 518 is short, there will be
insufficient loss coupled to the transmission line portions
514.
The power absorbing effectiveness of the structure 500 can be
characterized by measuring the percent power absorbed, Pa, for a
given unit length, Lrb. Considering connection 506b as an input and
506a as an output, Pa is defined as:
where Pr is the percent power reflected at the input 506b, and Pt
is the percent power transmitted at the output, 506a, considering
the incident power, Pi, at the input 506b, as one hundred
percent.
A prototype of power absorbing structure 500 on a BeO substrate
with resistors 516, 518 of 50 ohm per square was simulated at 30
GHz. 68.5% of the incident power was absorbed. The relevant
dimensions of the prototype are listed in Table II.
The percent power absorbed for the bias structure 350 (discussed
previously with reference to FIG. 13) having comparable dimensions,
was simulated to be about 50.8%. It is apparent that the meandering
bias structure 500 is a more area efficient power absorbing
structure.
Selection of the dimensions for the structure 500 are constrained
by several factors: 1) the area available, e.g., the maximum height
and width allowable; 2) the photolithographic process limitations
(e.g. minimum line width and spacing for the given process). For a
given process, the minimum line width and gap spacing determines W,
Dg and Ws1. X2 is then constrained to be X1+2*Dg. With a given
structure height, the other parameters Wg, Y2 and Y1 must be
selected. Given a practical minimum limit for Wg of about 3 mils,
the only design parameter to be selected is to apportion the
lengths of Y2 and Y1. Simulations using a commercial
electromagnetic simulator software package, such as "IE3D",
available from Zeland Software, Fremont, Calif., "EM", from Sonnet
Software, New York, N.Y., or others, may be used to optimize this
apportionment.
While the foregoing detailed description has described several
embodiments in accordance with this invention, it is to be
understood that the above description is illustrative only and not
limiting of the disclosed invention. It will be appreciated that it
would be possible to modify the size, shape, appearance and methods
of manufacture of various elements of the invention or to include
or exclude various elements within the scope and spirit of this
invention. Different spurious mode suppression structures are made
possible by changing the patterns of the adjacent coplanar
resistive films and/or the conductors. The conductor and resistor
patterns of this invention may be disposed in non-uniform planar
topologies such as spiral, circular, log periodic, exponential,
meandering, serpentine, semi-circular arc segments and the like.
Thus the invention is to be limited only by the claims as set forth
below.
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